Cross-mounted mask changer with thickness monitoring

ABSTRACT

A single substrate is exposed in a single pumpdown to vapor deposition from a plurality of evaporation sources. A photo processed mask for each source is mounted on a linearly movable carriage, moving the masks sequentially beneath the substrate for contact with it. The carriage, the substrate holder assembly, a shutter for selecting vapor sources, as well as the sources themselves are all mounted and contained within a Pyrex cross, sealed at each of its ports to sustain a vacuum. A deposition-monitoring crystal attached to the substrate holder is shielded by plates having different sized holes in the various mask-changer positions. The diameter of the openings in the various shields are adjusted to the specific density and the desired thickness of the vaious evaporants. Monitoring is possible in mask changer devices where extended evaporation of successive layers of different materials of various thickness is required.

United States Patent [1 91 Harel [4 1 July 24, 1973 CROSS-MOUNTED MASK CHANGER WITH THICKNESS MONITORING [75] Inventor: Abraham Harel, Givataim, Israel [7 3] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, D.C.

[22] Filed: Nov. 3, 1972 [21] Appl. No.: 303,437

Related US. Application Data [63] Continuation-impart of Ser. No. 170,501, Aug. 10,

[52] US. Cl. 118/8, 118/48 [51] Int. Cl. C23c 13/08 [58] Field of Search 118/7, 8, 48-495; l17/106 -107.2

[56] References Cited UNITED STATES PATENTS 2,621,624 12/1952 Chilowsky 118/49 X 2,745,773 5/1956 Weimer 118/49 X 3,023,727 3/1962 Theodoseau et al.... 118/49 X 3,117,025 1/1964 Learn et al. 118/49 3,206,322 9/1965 Morgan 118/49 X 3,207,126 9/1965 Byron 118/49 3,228,794 1/1966 Ames 118/49 X 3,238,918 3/1966 Radke et al 118/49.1

3,302,609 2/1967 Walker et al. 118/49 3,312,572 4/1967 Norton et al. 118/49 X 3,383,238 5/1968 Unzicker et a1. 118/49 X FOREIGN PATENTS OR APPLICATIONS 1,058,050 2/1967 Great Britain 118/49 Primary Examiner-Morris Kaplan Attorney-Harry A. Herbert, Jr. et al.

[57] ABSTRACT A single substrate is exposed in a single pumpdown to vapor deposition from a plurality of evaporation sources. A photo processed mask for each source is mounted on a linearly movable carriage, moving the masks sequentially beneath the substrate for contact with it. The carriage, the substrate holder assembly, a shutter for selecting vapor sources, as well as the sources themselves are all mounted and contained within a Pyrex cross, sealed at each of its ports to sustain a vacuum.

A deposition-monitoring crystal attached to the substrate holder is shielded by plates having different sized holes in the various mask-changer positions. The diameter of the openings in the various shields are adjusted to the specific density and the desired thickness of the vaious evaporants. Monitoring is possible in mask changer devices where extended evaporation of successive layers of different materials of various thickness is required.

6 Claims, 5 Drawing Figures PATENTED 3.747. 558

SHEU 3 [IF 3 CROSS-MOUNTED MASK CHANGER WITH THICKNESS MONITORING This application is a continuation-in-part of my patent applications, Ser. Nos. 170,501 and 170,531, both filed on Aug. 10, 1971 and now abandoned, which are continuations-in-part of my patent applications Ser. Nos. 858,401 and 858,402, respectively, originally filed on Sept. 16, 1969 and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a mask changer mounted in vacuum in a glass cross and, more particularly, to an automatically cycled and monitored device for vapor deposit of metal vapor or other evaporable material on a substrate surface that is partially and sequentially masked to establish a desired conductivity pattern. The pattern varies cyclically as the machine traverses a set path. External means are provided for lateral movement of the carriage and shutter, and for vertical movement of the substrate holder assembly. Precision means are provided for the engagement of mask frame and substrate holder. The deposition monitoring is accomplished by means of a crystal attached to the substrate holder where there is extended deposition and mask changing in a single vacuum operation and where successive layers of a number of different materials in a wide range of thicknesses is required.

For example, thin film transistors have been fabricated to date predominantly through vacuum evaporation and deposition of all device layers by mechanical masks.

In the field of thin-film vacuum-deposition, extensive use is made of thickness monitoring by means of a crystal oscillating in the vacuum system. Some of the evaporant is permitted to deposit on the crystal, thus altering its frequency, which is mixed with an external crystal oscillator, resulting in a low-beat frequency whose variations are proportional to the mass, hence the thickness of the deposited film. This holds true only within a certain linear range. A problem arises when the crystal is used to monitor successive depositions during an uninterrupted evaporation run carried out with a mask changer. Some films may be a hundred times thicker than others, yet the linear range must be maintained throughout the accumulated deposition, which may exceed 10,000 A., and on the other hand, the sensitivity must be high in order to accurately mea' sure very thin films, which may be well under 100 A.

SUMMARY OF THE INVENTION The object of the present invention is the provision of means for precision mask-changing and other required instrumentation for a vacuum evaporator housed in a 6-inch diameter glass cross.

Another object of the invention is the provision of a device for multiple material deposition in one vacuum system on a common substrate during a single pumpdown by linear movement ofa plurality of mask frames with reference to the substrate.

Still another object of the invention is the provision of precision means for the engagement of a substrate holder sequentially with a series of mask frames.

A further object of the invention is the accomplishment of monitoring of thin film thickness by means of a crystal in successive depositions during an uninterrupted evaporation run, and carried out with a maskchanger.

A standard Pyrex or glass cross is mounted in a rigid aluminum reference frame. The four ports of the cross are vacuum sealed to adjacent openings provided in the aluminum frame. A ten position linear mask changer carriage is supported at its ends by the frames extension, and is mounted for linear movement within the vacuum. It moves by screw means operated externally.

A substrate holder assembly is supported from the top port of a glass cross, and is capable of vertical move ment, and operated by means located outside the vacuum. The substrate holder assembly is alsomounted for a certain amount of linear movement. The masks can thus be moved sequentially into engagement with the substrate.

Appropriate drives and monitoring means are provided for moving the mask carriage, and also for controlling movement of a shutteror shield element which is mounted below the mask carriage. The shutter is provided with openings for placing a selected material evaporation source into operating relationship with the substrate position. Ten masks are provided for a single pumpdown and ten evaporation sources, either fixed or rotary.

Thick Viton seals or equal are used to clamp a coarse Pyrex cross at each of its four ports against a rigid aluminum reference frame.

In the present invention, monitoring of the rate of deposit and the thickness of the film is accomplished by placing a shield at each of the mask positions. Each shield is provided with an opening whose size is appro priate to the amount of deposit desired to be made on the substrate when masked by a given mask. Each mask, therefore, has an appropriate size for the openingin its individual shield. The opening is located under the center of the crystal when the appropriate mask is moved beneath the substrate holder. A small opening is used when a thick deposition is desired, and a large hole for a thin deposition. Monitoring is effected by the change in frequency of oscillation of the crystal in vacuum. The frequency change is proportional to the mass deposited; that is, proportional to the product of the specific density of the evaporant, the thin film thickness, and the square of the hole diameter. Consequently, if the diameter of the large hole is one-fourth inch, and the diameter of the small hole is onesixteenth inch, the frequency change where the large hole is used is 16 times greater than that through the smaller hole assuming that the depositions are of equal density and thickness. Very thin films can be monitored with a sensitivity (HZ/A.) which is 16 times greater than that of thicker films while an accumulation of these thicker films on the crystal does not exceed the mass which limits the linear range.

These and other objects, features and advantages will become more apparent after considering the description that follows taken in conjunction with the attached drawing and appended claims.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing in perspective of the linear mask change and evaporation system mounted in an aluminum frame. The glass cross for providing vaduum has been removed to show other details;

FIG. 2 is a cross sectional view of a standard glass cross;

FIG. 3 is a side view of a substrate holder and crystal holder. The view looks in the direction of the linear movement of the mask changer;

FIG. 4 is a top view of the crystal and crystal holder looking in the direction of the arrow 2 of FIG. 1; and

FIG. 5 is an enlarged detail showing the relative locations of the mask changer carriage and shutter as well as the evaporation source, crystal and crystal shield.

DESCRIPTION OF THE PREFERRED EMBODIMENT The frame 10, of aluminum or equal, provides the structural strength and the mechanical reference surfaces. A standard Pyrex cross 11 floats" between Viton gaskets which provide a vacuum seal between each one of its four ports and a similar opening in the aluminum frame 10. For convenience of disclosure the cross has been omitted from FIG. 1.

The right port of the cross leads to a high-vacuum pump (not shown), through a frame extension of a three-way valve which also supports and aligns the mating end 12 of the mask changer carriage 20. The top port provides access to the support 13 and elevator means 15 for the substrate 16, which is mounted in frame 18. The vertical movement thus afforded is accomplished by means of a bellows sealed mechanical coupling, shown at 24 in FIG. 1, which connects to the elevator elements 13 and 15.

The substrate frame 18 also has a degree of lateral movement with respect to the support element 13. This is accomplished by a sliding connection shown at 21, which insures precision engagement of substrate and mask. Other means for insuring this very important precision engagement will be described later in more detail.

The left port of the frame extension supports the main body of the mask changer carriage 20. This port, and the left opening 28 in the frame permits the insertion and removal of the carriage 20. It also provides for the movement of the masks 22 and the shutter 30. These elements are moved individually to the right or to the left. This is accomplished by two bellowssealed rotary mechanical couplings, including internal drives. These couplings are indicated schematically at 32 in FIG. 1.

The bottom port of the aluminum frame supports and provides access to the evaporation crucibles 35, through the bottom port of the Pyrex cross 11.

Each photo processed metal mask 22 is clamped onto a stainless steel frame 34 by means of four precision pins 36, mounted one at the corner of each frame. The carriage 20 carries as many as I0 of these frames 34, each one supporting a photo processed mask 22.

The carriage 20 is mounted on guide rods 37 upon which the carriage moves to the right or to the left. The screw 38, for accomplishing this movement, is coupled to an externally located drive 32.

A shield 41 supports the entire mask assembly and protects it from undesired evaporation deposition. The movement of the shutter 30, as noted above, is controlled by an externally driven means. An opening 42 in the shutter 30 provides the path for depositional material evaporating from the crucible 35 to the substrate 16.

The substrate assembly 17, comprising the substrate 16 and the frame 18, can move up or down and has limited movement to the right and to the left to insure Precision alignment of mask and substrate, and to align directly over a chosen evaporation crucible 35.

The substrate holder 18, normally mounted at the bottom of the holder assembly, and facing the mask, serves also to retain a clamp means for a quartz crystal 44. For precision engagement of substrate and mask, the frame 18 carries four tapered holes 46, two of which are guidance holes that engage the two long pins 36, mounted on each of the mask frames 34. The other two of the four holes 46 are extremely precise openings which provide the precision alignment with two short precision pins 36. By this means precision registration is obtained between the substrate and any mask.

Located adjacent each mask frame 34 there is an interchangeable shield 48 for use with a deposition monitor. Standard types of controlled mass monitor are available. They include: crystal controlled, optically controlled interference monitor, or electrically controlled conductance monitor.

Ten fine wires (not shown) may be attached to the rear of the substrate assembly. These make contact to the side of the substrate 16 and provide electrical monitoring of the deposition devices during or after evaporation while in the vacuum, or during heating in the vacuum. A heater 50 is provided for this purpose above the substrate and within the substrate assembly.

Non-perpendicular evaporations as needed for contacts to the sides of the substrate are accomplished by a raised crucible at any of the standard crucible positions (see 35 in FIG. 1). These crucibles may be fixed, or they may be replaced by a rotating assembly of six crucibles. All crucibles are resistance-heated, aluminacoated and are separated from each other by shields (not shown). Vapor from the selected vapor source 35 is thus directed through the shutter opening 42 (see FIG. 5) and through the appropriately sized opening in the shield 48 provided for each mask position to the appropriately masked substrate 16 and crystal 44.

As noted, the amount of deposition on the crystal 44 is a monitoring means for thin film thickness monitoring. Any deposition on the crystal increases the mass, decreasing its oscillation rate. A large opening is chosen for thin film deposit, and small opening when a thicker film is required.

The oscillation rate of the crystal 44 is an indication only. It is conceivable, however, and wholly within the scope of the invention that the oscillation may itself be utilized in a servo device to govern the deposition.

Referring now to FIGS. 3 and 4, the crystal means for thickness monitoring is shown in detail. The carriage 20 supported by and laterally movable along guide rods 37 is mounted in the vacuum system as hereinbefore described.

A substrate 16 upon which multiple depositions are to be made in sequence is mounted in a substrate holder 18, and lies at the bottom of a substrate holder assembly for precision contact with the masks 22, mounted in mask frames 34 which are brought in sequence under it. The masks 22 have been photoprocessed for deposition on the substrate of an evaporant in a designed pattern from one of a plurality of evaporation sources 35 located beneath the carriage 20 and at the base of the vacuum system.

The substrate assembly includes an elevator element 15 attached to and operated by a bellows (not shown) located outside the vacuum system. It includes also a tongue and groove engagement 17, by which the housing 19 and the substrate frame 18 are attached, and by which means the substrate frame 18 is allowed lateral motion. A heater 50 is also a part of the substrate assembly. The shutter 30 is mounted below the carriage 20 and is movable to the right and left independently of the carriage 20.

The crystal 44 is mounted in a crystal holder 62 at-- tached by appropriate bracket means 64 to the substrate holder 18. Each mask frame 34 has a shield 48 located adjacent to it in the carriage 20. Each shield 48 has an opening 65 whose size is designed appropriately for a selected mask and a selected source, and in conformity with the thickness of the film desired to be made on the substrate by any given mask.

The crystal 44 as noted, is carried by the substrate holder 18 in a frame 66. When a mask 34 is moved to a position beneath the substrate 16 and into operating contact with it, the shield 48 with its opening 65 sized appropriately to the individual mask in use, is moved into a position directly beneath the crystal 44.

The oscillation rate of the crystal 44 in vacuum is modified by the mass of the material deposited upon it. The rate variation therefore becomes an indication measurably through electrical lead 68 of the rate of deposit upon the crystal 44and therefore upon the mask substrate as well. While the device as shown is an indicator only, it is conceivable that within the scope of the invention, the oscillation and its variations may itself be utilized in a servo device to govern the deposition, either to terminate it or increase or decrease its rate.

FIG. 5 shows the relative position of crystal 44, crystal shield 48 and mask 22. It also shows the possibility of raising the level of an individual source 35.

MODE OF OPERATION By the utilization of the hereinbefore described invention, double-gated compatible complementary devices can be co-fabricated in one vacuum system on a common substrate during a single pumpdown by evaporation alone through permanent photo-processed metal mask exclusively. Both pand nenhancement types can be made concurrently using common processing steps; both having insulated gates under and above the semiconductor channel, both having equal source-drain patterns evaporated through highprecision collimating metal masks, and both having similar current-voltage characteristics. With proper equipment, including a high-capacity low-pressure vacuum system, efficient evaporation crucibles, the welldesigned mechanical mask changer hereinbefore de scribed with good mechanical masks, complementary devices can be co-fabricated by using the foregoing method with considerable ease and flexibility andwith good results. This is made possible by the freedom in choice of evaporation materials and evaporation sequences inside the vacuum chamber without any exposure of the substrate and the device layers to atmosphere or to other contaminating materials during the various fabrication steps of the typical complementary thin film transistor device.

Successive masks are automatically aligned by means of the mechanical mask changer mounted inside the Pyrex cross vacuum chamber according to the invention. The precision obtained is 0.0002 inch as compared with a 0.0001 inch precision that can be achieved by manual alignment with a high power microscope either inside the vacuum chamber, with an associated increase in complexity, or outside the vacuum system, with unavoidable oxidation and contamination during the process. The six-inch Pyrex cross, which compared to a bell-jar, combines small volume with convenient access, houses the evaporation crucibles as well as the linear mechanical mask changer with its associated instrumentation especially useful for the fabrication of complementary thin film transistors.

The mask assembly on the mask frame is shown in greater detail in FIG. 5. As already mentioned, the photoprocessed metal mask 22 is clamped to the mask frame 34 by means of vertical precision pins 36. In addition to the precise alignment of these pins by means of the precision holes 46 in the substrate holder 18 of a series of masks with respect to the substrate held in the substrate holder 18, this method provides a convenient assembly (and disassembly) means of the auxiliary masks onto the mask frame 34 which are essential for achieving improved deposition results.

Monitoring of the film thickness as it is being deposited is accomplished by a crystal mass-monitoring system. The system includes two crystal oscillators, one inside shown at 44 and one outside the vacuum system (not shown), a mixer, a frequency meter for measuring the beat frequency, and an adjustable power supply. The external crystal can be replaced by any one of many crustals of different frequencies to produce the desired beat-frequency range. The internal crystal 44 is mounted in the crystal holder 62 (FIG. 3) which is attached to the substrate holder 18 and is exposed to the evaporation source 35 when the shutter 30 is open.

The change in mass of the exposed crystal 44 creates a change in its frequency and results in a changed beat frequency. This, in turn, can be correlated with the independently measured thickness of the deposited film on the substrate 16, usually optically, through interferometry. In the linear range, advantage is taken of interpolation as well as relative densities of different evaporants.

The deposition sequence of a typical contentaddressable Memory array is long and contains a wide range of film thickness. Proper use can therefore be made of the crystal shields 48, associated with each mask 34, which have the appropriate size holes 65. The two obvious extremes are the heavy lands deposition of aluminum and gold which must use the small holed (one-sixteenth inch diameter, for example) shield, and the light semi-conductor deposition of cadmium selenide and tellurium' which must use the large-holed (one-quarter inch diameter, for example) shield, resulting in a reasonable frequency change. Thus, the operating range of the oscillating crystal monitor is not exceeded throughout the uninterrupted series of accumulated evaporations and retains its high sensitivity to very thin films.

Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims:

Having thus described my invention, what I desire to secure by Letters Patent of the United States is:

1. In combination, a mask changer for sequential deposition of vapor in a vacuum on a single substrate through a plurality of masks;

said mask changer being positioned in an evacuated glass cross having right and left, top and bottom ports, a frame and means for providing a vacuum seal beneath said frame and each port of said cross;

said mask changer comprising a carriage having linear movement within said vacuum;

a substrate holder assembly mounted for vertical movement and limited lateral movement with reference to said carriage;

mask frames mounted on said carriage in linear relationship with reference to each other and to said substrate holder;

means for raising and lowering said substrate holder for allowing movement of said carriage for placement of individual mask frames beneath said substrate holder;

cooperating means on said substrate holder and each of said masking frames for precision engagement of said substrate holder with each of said masking frames; and

a wide range thickness monitoring device positioned on said mask changer for monitoring a successive plurality of vapor depositions of widely varying thickness;

said monitoring device comprising at least one vapor source for deposit of vapor onto said substrate;

a crystal located in the vapor path to receive vapor from the vapor source each time a deposition is made on the substrate;

a laterally movable shield interposed between the vapor source and said crystal, said shield having openings therein in alignment with said crystal at each mask position;

each of the openings in said shield being of a preselected size according to the thickness required for the vapor deposition at the particular mask position, whereby the oscillation rate of said crystal is decreased in substantially linear proportion to the increase in its massas each successive deposition is made and the thickness of each deposition on the substrate is thereby indicated throughout a wide range of thickness.

2. In a mask change as claimed in claim 1, a substrate in said substrate holder and a mask in each of said masking frames a plurality of evaporation sources located in said vacuum area and below said carriage, a shutter located between said sources and said carriage, an opening in said shutter, means for moving said shutter to direct vapor from a selected one of said sources through said opening to said substrate, each mask being appropriately photoprocessed for patterned deposition of material from one of said sources onto said substrate.

3. In a mask changer as claimed in claim 2, guide rods supporting said carriage, means for moving said carriage to the right and to the left, said means comprising a screw operated from outside the left port of said cross and through a bellows coupling.

4. In a mask changer as claimed in claim 2, means for providing vertical movement of said substrate assembly, said means being operated from outside said vacuum area and through a bellows coupling located outside the top port of said cross.

5. In a mask changer as claimed in claim 1 wherein the openings in said shield are smaller for the thicker vapor depositions and larger for the thinner vapor depositions.

6. In a mask changer as claim in claim 5 wherein the ratio of larger to smaller openings in said shield is on the order of 4 to l. 

1. In combination, a mask changer for sequential deposition of vapor in a vacuum on a single substrate through a plurality of masks; said mask changer being positioned in an evacuated glass cross having right and left, top and bottom ports, a frame and means for providing a vacuum seal beneath said frame and each port of said cross; said mask changer comprising a carriage having linear movement within said vacuum; a substrate holder assembly mounted for vertical movement and limited lateral movement with reference to said carriage; mask frames mounted on said carriage in linear relationship with reference to each other and to said substrate holder; means for raising and lowering said substrate holder for allowing movement of said carriage for placement of individual mask frames beneath said substrate holder; cooperating means on said substrate holder and each of said masking frames for precision engagement of said substrate holder with each of said masking frames; and a wide range thickness monitoring device positioned on said mask changer for monitoring a successive plurality of vapor depositions of widely varying thickness; said monitoring device comprising at least one vapor source for deposit of vapor onto said substrate; a crystal located in the vapor path to receive vapor from the vapor source each time a deposition is made on the substrate; a laterally movable shield interposed between the vapor source and said crystal, said shield having openings therein in alignment with said crystal at each mask position; each of the openings in said shield being of a preselected size according to the thickness required for the vapor deposition at the particular mask position, whereby the oscillation rate of said crystal is decreased in substantially linear proportion to the increase in its mass as each successive deposition is made and the thickness of each deposition on the substrate is thereby indicated throughout a wide range of thickness.
 2. In a mask change as claimed in claim 1, a substrate in said substrate holder and a mask in each of said masking frames a plurality oF evaporation sources located in said vacuum area and below said carriage, a shutter located between said sources and said carriage, an opening in said shutter, means for moving said shutter to direct vapor from a selected one of said sources through said opening to said substrate, each mask being appropriately photoprocessed for patterned deposition of material from one of said sources onto said substrate.
 3. In a mask changer as claimed in claim 2, guide rods supporting said carriage, means for moving said carriage to the right and to the left, said means comprising a screw operated from outside the left port of said cross and through a bellows coupling.
 4. In a mask changer as claimed in claim 2, means for providing vertical movement of said substrate assembly, said means being operated from outside said vacuum area and through a bellows coupling located outside the top port of said cross.
 5. In a mask changer as claimed in claim 1 wherein the openings in said shield are smaller for the thicker vapor depositions and larger for the thinner vapor depositions.
 6. In a mask changer as claim in claim 5 wherein the ratio of larger to smaller openings in said shield is on the order of 4 to 